Master cylinders of known type comprise a body pierced with a bore in which is mounted to slide with sealing at least one piston connected to a control member, the piston dividing the bore into a supply chamber supplied by a reservoir of brake fluid by means of a supply duct made in the body of the master cylinder and a work chamber in communication with brakes situated at the wheels. The piston in its central part has a passage for communication between the supply chamber and the work chamber, this passage being fitted with a valve that is open at rest and that shuts off the passage between the supply chamber and the work chamber during the braking phase. The piston also has a piston head facing toward the work chamber and a piston shank receiving the control member of the master cylinder, the piston shank being connected to the piston head by a shoulder running radially outward and forming a surface via which the piston bears, at rest, against a thrust ring, the thrust ring surrounding the piston shank.
At the end of the braking phase, the piston, under the action of a return spring arranged in the work chamber, is pushed back toward the control member and strikes the thrust ring, causing a noise annoying to the driver who might fear that his braking system is malfunctioning. At the same time, the brake fluid flows from the supply chamber toward the tank between the body of the master cylinder and the thrust ring.
This noise problem occurs particularly in electro-hydraulic braking systems comprising a master cylinder actuated by a control member, equipped with means for simulating the pedal feel and serving in normal operation to simulate the reaction of the braking circuit, and a computer detecting the movement of the control member and generating a signal commanding a hydraulic pump to send a certain volume of brake fluid to the brakes. What happens is that electro-hydraulic systems are more sensitive to this noise because, unlike conventional braking systems, they have no pneumatic brake booster positioned between the brake pedal and the master cylinder and which, at the end of braking, slows the return of the piston of the master cylinder and therefore attenuates the noise. In addition, in the case of a tandem master cylinder, it is particularly the piston of the primary hydraulic circuit which is responsible for this noise.
In order to reduce this noise, it has been proposed, for example, that the thrust ring be covered with an elastomer so as to damp the impact of the piston against the washer. However, this solution is not entirely satisfactory.
Unpublished French Patent FR 01/13 073 proposes, by way of a solution, arranging a flow restrictor in the supply duct to slow the flow of the brake fluid from the supply chamber to the reservoir as the piston returns, and thus forming a hydraulic damper. However, this solution has a high cost because it entails an additional part. In addition, fitting this restrictor in the supply duct carries the risk of causing possible contamination which may disturb the operation of the master cylinder, for example the sealing of the sliding of the master cylinder in the bore.